Tyrosine Sulfation Is a Trans-Golgi-Specific Protein Modification Patrick A
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Tyrosine Sulfation Is a trans-Golgi-specific Protein Modification Patrick A. Baeuerle and Wieland B. Huttner Cell Biology Program, European Molecular Biology Laboratory, 6900 Heidelberg, Federal Republic of Germany Abstract. The trans-Golgi has been recognized as heavy chain detectable by sulfate labeling, as well as having a key role in terminal glycosylation and sorting the heavy chain sulfated in a cell-free system in the of proteins. Here we show that tyrosine sulfation, a absence of vesicle transport, already contained galac- frequent modification of secretory proteins, occurs tose and sialic acid. Third, sulfate-labeled IgM moved specifically in the trans-Golgi. The heavy chain of im- to the cell surface with kinetics identical to those of munoglobulin M (IgM) produced by hybridoma cells galactose-labeled IgM. Lastly, IgM labeled with sul- was found to contain tyrosine sulfate. This finding al- fate at 20~ was not transported to the cell surface at lowed the comparison of the state of sulfation of the 20~ but reached the cell surface at 37~ The data heavy chain with the state of processing of its suggest that within the trans-Golgi, tyrosine sulfation N-linked oligosaccharides. First, the pre-trans-Golgi of IgM occurred at least in part after terminal glycosy- forms of the IgM heavy chain, which lacked galactose lation and therefore appeared to be the last modifica- and sialic acid, were unsulfated, whereas the trans- tion of this constitutively secreted protein before its Golgi form, identified by the presence of galactose and exit from this compartment. Furthermore, the results sialic acid, and the secreted form of the IgM heavy establish the covalent modification of amino acid side chain were sulfated. Second, the earliest form of the chains as a novel function of the trans-Golgi. major function of the Golgi complex is the covalent expect that the sulfation reaction should occur in one or more modification of proteins destined for lysosomes, of those compartments that are part of the secretory pathway, secretory granules, and the plasma membrane. Gol- i.e., the rough endoplasmic reticulum, the Golgi complex, gi-specific processing of asparagine-linked oligosaccharides and the various transport vesicles and storage granules. In- is particularly well characterized (for review see Kornfeld deed, a recent study indicated that after subcellular fraction- and Kornfeld, 1985; Farquhar, 1985). These reactions in- ation tyrosylprotein sulfotransferase activity is highest in clude (a) the specific removal of carbohydrate units, (b) the Golgi-enriched membrane fractions and that the active site addition of new carbohydrates, and (c) the addition of resi- of this membrane-bound enzyme is oriented toward the Gol- dues other than carbohydrates, such as phosphate to N-linked gi lumen (Lee and Huttner, 1985). oligosaccharides. The detailed characterization of these mod- Among the subcompartments of the Golgi, the trans-most ification reactions has contributed significantly to the under- structures, referred to as the trans-Golgi network (Grifliths standing of the structural and functional organization of the and Simons, 1986), have recently received a great deal of at- Golgi complex, which is currently viewed to comprise at tention. The trans-Golgi network has been proposed to have least three distinct subcompartments, the cis-, medial, and a key role in the sorting of lysosomal, plasma membrane, trans-Golgi (for reviews see Farquhar and Palade, 1981; Tar- and secretory proteins into membrane vesicles that transport takoff, 1982; Dunphy and Rothman, 1985; Farquhar, 1985). them to their specific destination. To understand the molecu- Compared with the information about the localization of pro- lar mechanisms involved in these sorting processes, it would cessing reactions involving N-linked oligosaccharides in the be helpful to identify protein components specific to the Golgi subcompartments, little is known about the localiza- trans-Golgi network. Such protein components would pro- tion of amino acid side chain modifications in the specific vide new markers for the trans-Golgi network that could subcompartments of the Golgi. be used for a more refined description of this compartment Recent work has indicated that the sulfation of tyrosine and that could also be employed for its isolation. Further- residues is a widespread protein modification and that secre- more, the characterization and comparison of several differ- tory proteins are the major physiological substrates for the ent trans-Golgi network-specific proteins may help to iden- sulfating enzyme(s) (Huttner, 1982; Hille et al., 1984; for re- tify domains on these proteins that are involved in their view see Huttner and Baeuerle, 1987). One would therefore specific localization and retention in the trans-Golgi net- Dr. Baeuede's present address is Whitehead Institute, Nine Cambridge Cen- work. Here, we report, using IgM as a model protein, that ter, Cambridge, MA 02142. tyrosine sulfation specifically occurs in the trans-Golgi and The Rockefeller University Press, 0021-9525/87/12/2655/10 $2.00 The Journal of Cell Biology, Volume 105 (No. 6, Pt. 1), Dec. 1987 2655-2664 2655 apparently is the last modification of this secretory protein a 10% CO2 atmosphere at 370C. Immediately after resuspension and after before its exit from the trans-Golgi. the times indicated, aliquots of the cell culture were removed and boiled in SDS-lysis buffer (see below), lgM was immunoaffinity-purified from the ly- sates as described in Immunoaffinity Purification of lgM and the radioactiv- Materials and Methods ity in the various forms of the IgM heavy chain determined by liquid scintil- lation counting after SDS-PAGE under reducing conditions. For pulse-labeling with pH]tyrosine, cells were resuspended at a den- Isotopes sity of 1 • l0 s cells/mi in labeling DME supplemented with 10% dialyzed FCS and allowed to equilibrate for 1 h in the incubator. Cells were then la- L-[3SS]methionine (carrier-free), L-[3,5-3H]tyrosine (1.5 TBq/mmol), L- beled for 10 rnin with 1.5 MBq/ml [3H]tyrosine, cooled on ice, and cen- [~C(U)]tyrosine (1.85 GBq/mmol), [3SS]sulfuric acid or sodium [3SS]sul- trifuged at 4~ for 10 rain at 150 g. The cells were resuspended in chase fate (carrier-free), L-[6-3H]fucose (0.93 TBq/mmol), v-[4,5JH(N)]galac - medium (complete DME, supplemented with 10% FCS) that had been pre- tose (1.7 TBq/mmol), and D-[2-3H(N)] mannose (0.89 TBq/mmol) were equilibrated in a 10% CO2 atmosphere at 37~ and distributed onto six obtained from New England Nuclear (Dreieich, FRG). 3'-Phosphoadeno- dishes that were immediately returned to the incubator. After the indicated sine 5'-phospho-[35S]sulfate (PAPS; 1 4.81 TBq/mmol) was kindly provided periods of chase, dishes were placed on ice. Cells and media were further by Christof Niehrs of this institution. processed as described in Immunoaffinity Purification of IgM. For pulse double-labeling, cells were resuspended at a density of 1 • Cell Culture 10s cells/ml in labeling DME supplemented with 0.1% dialyzed FCS and allowed to equilibrate for 1 h. Cells were then labeled with a mixture of 18.5 The hybridoma cell line studied is the clone 81-O4 of Sommer and Schach- MBq/ml [35S]sulfate and 12.3 MBq/ml [3H]galactose for 5 min in the incu- ner (1981), which secretes lgM directed against the cell surface antigen 04. bator. Thereafter the cell culture was immediately cooled on ice, and the Cells were grown at 37~ in complete DME (4.5 g/liter glucose) sup- cells were removed and collected by centrifugation at 4"C in a 50-ml tube plemented with 15% FCS (Gibco Laboratories, Karlsrnhe, FRG) in an at- (Falcon Labware). After a wash in ice-cold PBS, the cells were resuspended mosphere containing 10% C(h. For all labeling experiments, cells were at a density of 1 • 107 cells/mi in complete DME (supplemented with collected by centrifugation at 150 g for 5 min, washed once in ice-cold PBS, 0.1% FCS and 2 mM D(+)-galactose) that had been preequilibrated in a 10% and resuspended after centrifugation as described in the sections below. Un- CO2 atmosphere at 370C. Immediately after resuspension, two aliquots less indicated otherwise, labeling was performed in 3.5- or 5-cm petri dishes were removed and cooled on ice (zero time values). The cell suspension re- in a 10% CO~z incubator. For all labeling experiments we used labeling maining in the 50-ml Falcon tube was returned to the incubator and placed DME (sulfate-free and L-tyrosine-free) that was supplemented with 4.5 g/li- on a slowly rotating platform during the chase period. Further aliquots of ter glucose and 0.1-10% FCS dialyzed against 10 mM Hepes, pH 7.3 and the well-suspended cell culture were removed at the indicated times and im- 150 mM NaC1. To increase the efficiency of [35S]sulfate-labeling, a lower mediately placed on ice. Cells and media were further processed as de- concentration of L-methionine and L-cysteine (2% of normal) was used in scribed under Immunoaflinity Purification of IgM. some experiments (Baeuerle and Huttner, 1986). This had no detectable influence on protein synthesis as monitored by the incorporation of [3H]ty- rosine into proteins. 20~ Block Cells were resuspended in a 50-mi tube at a density of 1 x 107 cells/ml in Long-term Labeling labeling DME supplemented with 0.1% dialyzed FCS and allowed to equili- brate for 5 h on a slowly rotating platform in the incubator at 37~ The Cells were resuspended at a density of 1 x 107 cells/ml in labeling DME tube was then closed to preserve the atmosphere, removed from the incuba- supplemented with 0.1-10% dialyzed FCS.